Willy Wonka, I feel, suffers from rose-colored glasses. If you were a kid when you saw it (even though you may have been a kid years and years after it was created), you tend to like the film more than you would if you were just seeing it for the first time AFTER your childhood. I wasn't one of the people that saw Willy Wonka while I was a child. I saw it when I was a teenager. So to me, it fell flat. I'm sure its audience is justified in enjoying it, but I felt no sort of magical butterflies in my stomach by viewing it.
I also viewed Taxi Driver for the first time as a teenager. The grit of the film is part of its charm but it also makes it difficult to watch. I certainly wouldn't classify Taxi Driver as a film you just pop in when you're bored. Though, that's not necessarily a bad thing as not all films are to be taken so lightly. Both films have made their stamps on pop culture, and although Willy Wonka may be the more popularized of the two, I feel the R-rated bias is part of the reasoning behind that.
Taxi Driver.